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KINETICS, CATALYSIS, AND REACTION ENGINEERING Modeling of Hydrodesulfurization Catalysts. I. Influence of Catalyst Pore Structures on the Rate of Demetallization Layioye O. Oyekunle* and Oghenerobo B. Ikpekri Department of Chemical Engineering, University of Lagos, Lagos, Postal Code 101017, Nigeria
A mathematical model to describe catalyst deactivation during residuum hydrodesulfurization (HDS) has been applied to catalyst systems of different pore structures. Simulations were carried out on the hydrodemetallization (HDM) reaction, which usually leads to the steady accumulation of metals during HDS. Catalyst activity decays with time both linearly and nonlinearly depending on the pore structure. Predicted lifetimes for three different pore structures show that the catalyst can deactivate within 5-15 months. The results of the simulations indicate that better service life could be achieved with improved catalyst design involving pore structure modifications. Introduction Refiners have long been concerned with sulfur levels, and a number of refinery technologies have been developed to remove sulfur from petroleum products. Hydrodesulfurization (HDS) is a catalytic process in which the principal purpose is to remove sulfur from petroleum fractions in the presence of hydrogen. Sulfur can be present in crude oil as hydrogen sulfide (H2S); as compounds such as mercaptans, sulfides, disulfides, thiophenes, etc.; or as elemental sulfur. Each crude oil has different amounts and types of sulfur compounds, but as a rule, the proportion, stability, and complexity of the compounds are greater in heavier crude oil fractions. The petroleum refining industry is faced with higher quantities of residua, more heavy oils, and greater amounts of sulfur to remove. The book by Speight and his paper on hydroprocessing provide an extensive and detailed overview of the current technologies for the hydrodesulfurization of heavy oils and residua.1,2 The combustion of petroleum products containing sulfur compounds produces undesirables such as sulfuric acid and sulfur dioxide. Catalytic hydrodesulfurization (HDS) removes sulfur compounds from refinery product streams so as to reduce their eventual emission and the consequent serious environmental problems. The reactors used commercially for HDS and their operating conditions have been identified, and the fixedbed reactor has been widely used.1 The catalyst is contained in the reactor as stationary beds with feedstock and hydrogen either passing through the bed in a downward direction or encouraged to flow through the bed in a radial direction. The upflow expanded-bed reactor in which the catalyst remains loosely packed has been used for heavier feedstocks. In a typical catalytic hydrodesulfurization unit, the feedstock is deaerated and mixed with hydrogen, preheated in a fired heater (320-420 °C), and then charged under pressure (up to 70 kg/cm2) through a fixed-bed * To whom correspondence should be addressed. Tel.: (234) 01 5454891-2 ext 1839, (234) 01 4932660-1 ext 1839. E-mail:
[email protected].
catalytic reactor for middle distillates and heavy crude oil. Residual hydroprocesses might require hydrogen pressures on the order of 100-200 kg/cm2 at higher temperatures in the range of 370-425 °C.1,2 In the reactor, the sulfur and nitrogen compounds in the feedstock are converted into H2S and NH3. The reaction products leave the reactor and, after cooling to a low temperature, enter a liquid/gas separator. The hydrogenrich gas from the high-pressure separation is recycled to combine with the feedstock, and the low-pressure gas stream rich in H2S is sent to a gas-treating unit where H2S is removed. The clean gas is then suitable for use as fuel for the refinery furnaces. The liquid stream is the hydrodesulfurized product. The kinetics of HDS reactions has been fairly extensively studied, and apparent reaction orders have been reported for both petroleum fractions and residual oils. Most of the rate equations published are simple firstorder disappearance kinetics for petroleum distillates.5,9-11 Ancheyta et al.5 obtained apparent reaction orders in the range of 1.70-1.98 for middle distillates that exhibited an increase as the molecular weight of the feed was increased. Residual HDS is more complex, and reaction rate measurements performed using isothermal reactors have reported an order of between 1.5 and 2.0.5,9-16 The second-order activation energy was observed for sulfur and vanadium removal activity for Maya crude atmospheric residue (>345 °C).14 There has been an increased interest in the upgrading of heavy fuels, petroleum residuals, and synthetic fuels such as coal oil, shale oil, and tar sands. To achieve a high degree of refining, an active catalyst must be used. The catalysts most commonly used are derived from alumina supported on oxides of cobalt, molybdenum, and nickel consisting of CoO + MoO3 or NiO + MoO3. Catalyst properties are the key to the effective HDS of heavy feedstocks, and catalyst consumption is a major aspect of the HDS process. Heavy feedstocks usually require frequent catalyst replacement, and huge costs are incurred within a 1-year period for catalyst replacements.1,17 This calls for the development of catalysts with a high resistance to deactivation. Among the causes of catalyst deactivation in HDS is the accumulation of metals such as nickel (Ni) and
10.1021/ie049618y CCC: $27.50 © 2004 American Chemical Society Published on Web 09/10/2004
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vanadium (V) as metal sulfides and coke deposition on the catalyst. Deposition of metals on the catalyst results in the blocking of access to the active sites of the catalyst, and it is the major cause of reduced activity of the original catalyst. In recent years, hydrodemetallization (HDM) catalysts have been developed to protect the more active HDS catalysts. This involves the development of new HDM catalysts for primary treatment and the production of new HDS catalysts for secondary treatment.2,3,17 Catalyst deactivation studies have resulted in the development of better catalysts and improved reactor design. Progress is being made in the development of HDS catalyst for long-term continuous operation. Pretreatment catalysts (reaction accelerators) and HDS catalysts employed have resulted in improved catalyst physical properties. Various amorphous-type catalysts are being produced, and X-ray diffraction measurements are being carried out, resulting in additional improvements. Although experimental procedures to quantitatively describe the deactivation processes are available, the modeling approach has also received some attention.11,18-24 In the present work, a second-order reaction mechanism for HDS catalysts is considered in a model simulation. Models developed for the macro-, micro-, and random pore structures were used in the prediction of catalyst lifetime.
are generally performed using isothermal reactors, whereas in commercial HDS reactors, a temperature gradient of 15-25 °C is commonly reported, and the temperature is usually increased to compensate for catalyst deactivation.6,7 Temperature homogeneity can be improved by diluting the catalyst bed with inert materials.4,6 (iii) The pores have small sizes, and the flow is unidirectional and along the pore length only. (iv) Molecules diffuse rapidly into the pores, and radial concentration gradients are negligible. (v) The HDS/HDM reactions are second order and occur at 675 K and a pressure of 200 kg/cm2. The validity of these assumptions with respect to the actual process needs to be verified with experimental data and subsequently upgraded. Diffusion and reaction within the pores can therefore be described by the continuity equation
∂C ∂2C )D 2 +R ∂t ∂x
where D is bulk diffusivity and R is the reaction rate. Macropore Model. When the mean pore radius is above 100 Å, transport in the pellet is said to occur in the macropore region.25,26 Hence, bulk diffusion prevails in such pores. The effective diffusivity for a macropore disperse system is given by
DeM ) DM∈M2
Model Formulation and Description Main Assumptions. The heterogeneous catalyst system is made up of solid material with straight cylindrical pores through which the reactants and products diffuse. In the random pore model, the pellet consists of an assembly of small particles having a volume distribution of micro- and macropores, usually with a pore radius of about 100 Å used as the dividing point,25 and the molecular sizes of the reacting species in the HDM/HDS of residuum petroleum fractions range from about 25 to 150 Å.26 Lostaglio and Carruthers27 reported that catalysts for use in distillate HDS applications generally exhibit porous structures with much of their pore volume in pores smaller than 100 Å whereas most residual HDS catalysts have more open porous structures and exhibit significant pore volumes in pores greater than 100 Å in diameter. Currently, it is well-known that catalysts for residual HDM applications should have pores more concentrated in the region of 100-250 Å (about 70 vol %), with 20 vol % of pores >250 Å and the remaining 10% with sizes of 250 Å were 15% and 75%, respectively, whereas pores that were
